1. Field of Invention
Apparatuses consistent with the present invention relate to a light emitting diode (LED) driver, and more particularly, to an LED driver that can improve efficiency of power consumption.
2. Description of the Related Art
An LED is used as a backlight for a liquid crystal display (LCD), where plural arrays of LEDs are provided for each of three colors, Red, Green and Blue.
A conventional driver used to drive LEDs can be configured with a circuit as illustrated in FIG. 1. In the driver 1 of FIG. 1, when a switching field effect transistor (FET) 2 is in an ON state, the current supplied by power source 5 flows on an LED 6 through an inductor 3, whereas, when the switching FET 2 is in an OFF state, the current flows on the LEDs by way of the inductor 3 and a diode 4.
The driver 1 controls the current flowing on the LED (hereinafter shortened as “LED current”) to have a predetermined peak value (so called “analog dimming”), and controls the LED current to be ON or OFF, through the use of a pulse width modulation (PWM) signal, having a predetermined duty cycle (so called “PWM dimming”), thereby adjusting luminance of the light emitted by the LED 6.
The LED current controlled by the driver 1 may be shaped with approximately square waves as illustrated in FIG. 2. If the peak value of the square wave is approximate to the maximum rated current of the LED 6 and its minimum value is approximately 0, the LED current and the average luminance of the LED 6 by the LED current is as illustrated in FIG. 3, by averaging the square waves. Here, an X axis indicates LED current and a Y axis indicates LED light output.
The average luminance of the LED provided by the LED current, shaped with square waves, is represented in the form of a straight line B, connecting a starting point of X-Y coordinates to the luminance value of the LED corresponding to the maximum rated current of the LED.
However, when direct current that is approximately constant in magnitude (hereinafter referred to as “constant current”: see FIG. 2) flows on the LED 6, the luminance of the LED 6 by the LED current is the same as a curve A depicted in FIG. 3. In this case, a relationship between current and luminance is not linear, but shows an exponential function wherein the light output of the LED 6 is saturated if the LED current is larger than a predetermined magnitude.
Accordingly, even in the LED current or LED average current having the same magnitude, a difference D1 occurs between the luminance (A) and the average luminance (B) of the LED 6 respectively to the constant current and the square wave current, as depicted in FIG. 3. For example, where the duty cycle of the square wave is 50%, the difference in efficiency between the luminance (A) and the average luminance (B) of the LED 6 respectively to the constant current and the square wave current is about 15% in terms of power.
As a result, like PWM control under which the LED current is shaped with square waves, controlling the LED current which is a periodical direct current is less efficient than controlling the LED current which is the constant current, in terms of power consumption.